1. Field of the Invention
The present invention relates to an indium borate phosphor used mainly in a display of a computer terminal or projection tube, and a cathode-ray tube using the indium borate phosphor.
2. Description of the Related Art
An LA phosphor of "Cd.sub.5 Cl(PO.sub.4).sub.3 :Mn" is known as a conventional phosphor for a display tube of a computer terminal or the like. This phosphor has a high emission brightness and a relatively long 10% afterglow persistance of about 30 msec. When this phosphor is used in a display, it exhibits excellent characteristics. This phosphor however contains 50 wt % or more of toxic cadmium and is not suitable from viewpoints of working environments and pollutions. Strong demand has arisen for a phosphor having excellent characteristics and not containing Cd.
A phosphor of InBO.sub.3 :Eu,Tb is proposed as a phosphor which does not contain cadmium and has an afterglow property (Published Unexamined Japanese Patent Application No. 59-105075). A lamp phosphor of (In,Y,La)BO.sub.3 :Eu excited with an ultraviolet ray to emit orange light is proposed (Published Unexamined Japanese Patent Application No. 57-51783).
The present inventors made extensive studies and conducted experiments to improve current-brightness characteristics of such indium borate phosphors. The current-brightness characteristics of these phosphors were somewhat improved by adding a predetermined amount of samarium in each phosphor of this type.
According to the present inventors, the emission brightness of the InBO.sub.3 :Eu,Sm phosphor could be improved to some extent by adding a small amount of terbium. There is also proposed an indium borate phosphor in which indium is partially substituted with aluminum to improve emission brightness (Published Unexamined Japanese Patent Application No. 61-127784).
These improved indium borate phosphors have been frequently used in place of conventional LA phosphors and P-4 phosphors of [ZnS:Ag+(Zn.Cd)S:Cu] having a large content of Cd.
Since these indium borate phosphors tend to be subjected to current saturation with an increase in current, these phosphors are not deemed to have sufficient current-brightness characteristics as compared with the P-53 phosphor [Y.sub.3 Al.sub.5 O.sub.12 :Tb] and an ittrium silicate phosphor [Y.sub.2 SiO.sub.5 :Tb] in particular, phosphor films of these indium borate phosphors are discolored (burned) with a lapse of operation time. Since a phosphor film is irradiated with an electron beam having a high current density in particularly a projection tube and a display tube, the phosphor film is subjected to the current saturation and degradation (burning). Burning is a phenomenon occurring when one portion is repeatedly irradiated with an electron beam having a high current density. When the indium borate phosphor is irradiated with an electron beam, In as a constituting element of the phosphor and a small amount of additive such as Tb are reduced. For this reason, metals such In and Tb precipitate from the crystal lattice consisting of bonds of In and BO.sub.3. Therefore, discoloring and a decrease in emission brightness of the phosphor film occur.